Hydraulic clamps are disclosed in such patents as U.S. Pat. Nos. 3,948,502; 3,605,569; and 3,572,216 wherein a clamp arm is supported for rotational movement to a position for clamping and then moved rectilinearly to apply clamping pressure. In these patents a rectilinearly-moved piston is used to impart rotation of the clamp arm through the intermediary of an inclined cam. The structure shown in these patents is considered to be complex and difficult to manufacture economically. The present invention has for its purpose to employ a fewer number of component parts in a different and less complex combination; which enables a smaller swing clamp arm so as to enable clamping workpieces with clamp arms of minimum size; which provides for greater rigidity and strength.